A fuel cell refers to a type of electric power generator that converts chemical energy of a fuel into electrical energy in a fuel cell stack via an electrochemical reaction. Such a fuel cell is used for the supply of electric power to small-sized electrical/electronic devices, for example, portable devices, as well as for the supply of electric power to industrial and household appliances and to vehicles. With the increase in demand for high-efficient clean energy, the range of application areas of the fuel cell has been continuously increasing.
FIG. 1 is an exploded view illustrating the constitution of a unit cell of a general fuel cell stack.
As shown in FIG. 1, a unit cell of a general fuel cell includes a membrane electrode assembly (MEA) 12, which includes a polymer electrolyte membrane having hydrogen cation (proton) conductivity and catalyst layers, i.e. an air electrode (cathode) and a fuel electrode (anode), coated on both surfaces of the electrolyte membrane so as to permit reaction between hydrogen and oxygen, a pair of gas diffusion layers (GDLs) 20, which is disposed on the respective electrode of the membrane electrode assembly 10 in order to aid diffusion of reaction gas, and a separator 30, which is closely adhered to each of the gas diffusion layers 20 in order to supply the reaction gas. Although not illustrated in the drawings, a porous member, which is capable of dispersing and diffusing gas, may be interposed between each of the gas diffusion layers 20 and the separator 30.
The separator 30 functions to maintain the shape of the fuel cell stack by electrically connecting the membrane electrode assembly 10 to another one and supporting the same while preventing hydrogen and oxygen, which are reaction gases, from being mixed with each other.
Therefore, the separator 30 must have a dense structure so as to prevent the reaction gases from being mixed with each other. Further, the separator 30 must have excellent conductivity and improved strength so as to serve as a conductor and a supporter. Accordingly, a separator made of a metal material is primarily used.
The separator includes channel portions and land portions, which are respectively spaced equidistantly apart from each other and extend parallel to each other along the direction in which the reaction gas flows. The channel portion serves as a passage through which the reaction gas flows and a passage through which water generated by the electrochemical reaction of hydrogen and oxygen during the operation of the fuel cell (hereinafter, referred to as “generated water”) is discharged to the outside of the fuel cell stack.
If the water generated due to the reaction of the fuel cell is not discharged smoothly but remains in the fuel cell stack, it may locally impede diffusion and exhaust of the reaction gas, causing degradation of the performance and efficiency of the fuel cell.
When the passage defined by the channel portion is blocked by the generated water, the generated water may sometimes be discharged due to the differential pressure of the reaction gas. However, there is a problem in that the discharge of the generated water is not achieved smoothly in a low-current section because the gas flow rate is low and the differential pressure is not that large in the low-current section.
FIG. 2 is a view illustrating a conventional separator 30 for a fuel cell, which is structured such that a channel portion 31 and a land portion 32 are formed in the direction of gravity in order to make the flow direction of reaction gas identical to the direction of gravity. Such a structure may have an advantage in that generated water is discharged smoothly.
The generated water flowing through the separator functions to maintain the performance of the cell by maintaining the humidity of the reaction gas flowing into the cell at a proper level. However, if excessive generated water is discharged, the moisture within the cell is reduced and the interior of the cell becomes dry. As a result, the moisture content within the electrolyte membrane is lowered, and the performance of the cell is degraded.